Gulf of Maine Seasonal Sea Surface Temperature Update

Seasonal Patterns in Sea Surface Temperatures

Author

Adam Kemberling

Published

December 21, 2023

About the Updates:

Over the past decade, scientists at the Gulf of Maine Research Institute have led a body of research that highlights the rapid pace of warming in the Gulf of Maine. To keep you informed, we share seasonal updates about conditions in the Gulf of Maine.

Note About the Data: The figures in this report are created using remotely-sensed satellite data as part of publicly funded research efforts. Satellite SST data was obtained from NOAA’s National Center for Environmental Information (NCEI), with all maps and figures displaying NOAA’s Optimum Interpolation Sea Surface Temperature Data.

The Gulf of Maine Region

For analyses like these, it is important to be clear about the spatial extent that “defines” the Gulf of Maine (Figure 1), as different borders could produce different results. The spatial domain we use as the “Gulf of Maine” is displayed below. This area is consistent with previous seasonal and annual reports that scientists at GMRI have produced.

Fall Highlights

For this seasonal report we present an analysis of SST for fall (September 1, 2023 - November 30, 2023). During this season, the average SST for the Gulf of Maine was 57.25°F, making it the 14th hottest fall on record for the period of 1982-2023 — the period over which the satellite data used are available. This seasonal average temperature is 0.72°F above the 1991–2020 climatological reference period (CRP) Fall average of 56.53°F.

Weekly Temperatures

In Table 1 we highlight how the SST for each week this fall compares to the 1991-2020 CRP for the area shown in Figure 1. The observed SST, long-term average SST, and SST anomalies (i.e., departures from the long-term average SST) are shown.

SST departures from that long-term average were on average 0.72°F or more above normal during the fall. With weekly averaged SST anomalies ranging from -1.79°F to +3.85°F.

Table 1:
Average weekly sea surface temperatures in the Gulf of Maine
One-Week Period	Observed Temperature	Climatological Average	Temperature Anomaly
Table 1. Weekly Averaged Sea Surface Temperatures - Fall
(September 1, 2023 - November 30, 2023)
Sep 01 - Sep 02	63.3°F (17.4°C)	62.7°F (17°C)	0.661°F (0.367°C)
Sep 03 - Sep 09	64.4°F (18°C)	62.3°F (16.8°C)	2.17°F (1.21°C)
Sep 10 - Sep 16	64.7°F (18.2°C)	61.6°F (16.4°C)	3.13°F (1.74°C)
Sep 17 - Sep 23	59.3°F (15.2°C)	60.8°F (16°C)	-1.41°F (-0.785°C)
Sep 24 - Sep 30	58.3°F (14.6°C)	59.8°F (15.4°C)	-1.46°F (-0.813°C)
Oct 01 - Oct 07	58.5°F (14.7°C)	58.7°F (14.8°C)	-0.195°F (-0.108°C)
Oct 08 - Oct 14	57.9°F (14.4°C)	57.6°F (14.2°C)	0.349°F (0.194°C)
Oct 15 - Oct 21	57.6°F (14.2°C)	56.3°F (13.5°C)	1.26°F (0.699°C)
Oct 22 - Oct 28	57.4°F (14.1°C)	55.1°F (12.8°C)	2.34°F (1.3°C)
Oct 29 - Nov 04	55.8°F (13.2°C)	53.9°F (12.2°C)	1.86°F (1.03°C)
Nov 05 - Nov 11	53.6°F (12°C)	52.8°F (11.6°C)	0.764°F (0.424°C)
Nov 12 - Nov 18	51.7°F (10.9°C)	51.8°F (11°C)	-0.0555°F (-0.0308°C)
Nov 19 - Nov 25	51°F (10.5°C)	50.8°F (10.4°C)	0.21°F (0.117°C)
Nov 26 - Nov 30	50.2°F (10.1°C)	49.9°F (9.95°C)	0.264°F (0.147°C)
Data Source: NOAA OISSTv2 Daily Sea Surface Temperature Data.
Climatological Reference Period: 1991-2020.

Monthly Statistics

Table 2 shows monthly average SST for fall 2023, where we see each month was nearly 1°F or more above the 1991-2020 CRP. October showed the largest deviation from the long-term climatological average, with an average SST anomaly of 1.08 °F.

Table 2:
Average monthly sea surface temperatures in the Gulf of Maine
Month	Rank (1982-2023)	Observed Temperature	Climatological Average	Temperature Anomaly
Table 2. Monthly Averaged Sea Surface Temperatures - Fall
(September 1, 2023 - November 30, 2023)
Sep	14	61.8°F (16.6°C)	61.2°F (16.2°C)	0.611°F (0.339°C)
Oct	12	57.7°F (14.3°C)	56.7°F (13.7°C)	1.08°F (0.598°C)
Nov	16	52.2°F (11.2°C)	51.7°F (11°C)	0.457°F (0.254°C)
Data Source: NOAA OISSTv2 Daily Sea Surface Temperature Data.
Climatological Reference Period: 1991-2020.

How Does this Fall Compare?

2023 is the 14th warmest fall season observed in the Gulf of Maine during the 41 years we have satellite data to analyze, a departure from a recent trend in above average fall temperatures in the last decade. Seasonal SST in 2023 fell 2.75F when compared to 2022. This marks the largest single-year decline in year-over-year fall temperatures.

Figure 2: A ranking of the 10 warmest summer seasons for the Gulf of Maine in the satellite record (1982-2023). 2023 was the eighth warmest summer on record.

Seasonal Trends and Long-Term Global Context

The Gulf of Maine is an area of particular interest to the scientific community because of the remarkable rate of warming it has experienced in recent years coupled with its importance as a major driver for the regional economy.

When looking at average fall temperatures from the beginning of the satellite record in 1982 Figure 3, there is a clear long-term increase, with SST’s warming at a rate of 1.11°F per decade. This rate of seasonal spring warming is ~4x the rate that global ocean temperatures are warming (0.26°F per decade).

Figure 3: Average annual spring SSTs in the Gulf of Maine from 1982 through 2022 (black dots). The orange line indicates the trend for the full time series for the Gulf of Maine. The blue line indicates the trend for the full time series for the global oceans.

The unprecedented levels of warmth over the past decade or so are consistent with what researchers describe as a distinct regime shift in terms of the major influences on SSTs in the Gulf of Maine. The drivers of this (e.g., a potential weakening of the Atlantic Meridional Overturning Circulation, widening of the Gulf Stream, changes in the characteristics of the Labrador Current) have been well-documented in the peer-reviewed literature, including through research by GMRI scientists.

Sea surface temperatures are also impacted by atmospheric weather patterns which can act to amplify warming or cool surface waters. Sustained periods of low cloud cover and the consequent direct sunlight can rapidly warm the ocean at the surface. Winds can act as a fan, advecting heat away and cooling the surface. Storm systems and any strong and/or sustained winds will also cool surface temperatures through mixing, as wave action stirs up cooler water from beneath the warmer surface layer, dispersing the heat through more of the water column.

In September the Gulf of Maine was crossed by extratropical storm Lee, which weakened from hurricane strength as it crossed over Georges Bank on September 16th. The impacts of this storm on the region’s SSTs can be seen in the declines observed this fall as the storm passed through the region and the persistent lows that followed. Similar storm crossings have occurred in the past, including during 2019 when hurricane Dorian passed through the Gulf of Maine after working its way up the US East Coast. Each of these years had some of the fewest fall heatwave days seen in recent years (2019: 0 days, 2023: 6 days).

Marine Heatwave Conditions

The most commonly used definition of a “marine heatwave” (MHW) is when daily average SSTs exceeded the 90th percentile of a climatological (i.e., 30-year) average for at least 5 consecutive days. Gaps of 2 days or less in this threshold do not constitute a break in the MHW event.

Using this broadly accepted definition, the Gulf of Maine has experienced MHW conditions for 7% of this fall. September was the only month this Fall where SST was above the threshold of a MHW (90th Percentile) long enough to meet standards for a MHW, but only for a total of six days.

Figure 4: A timeseries of marine heatwave (MHW) conditions in the Gulf of Maine extending from January 1, 2023 through August 31, 2023. Black lines represent the long-term (i.e., 1991 – 2020) average SST, the 10th percentile, and 90th percentile for a given day in the Gulf of Maine; a solid line (red for marine heatwave or blue for a non-event) indicate the observed SST this year; red (above 90th percentile) and blue (below 90th percentile) shading illustrates how far the observed SST falls from the MHW threshold.

Presenting SST conditions in terms of anomalies ( Figure 5 ) as opposed to absolute values ( Figure 4 ) illustrates in greater detail the magnitude of MHW conditions throughout the fall. The most extreme daily SST anomalies for fall 2023 occurred towards the start of September, with temperatures building to over 3°F above the climatological average above the climatological average before rapidly dropping to cooler for the remainder of the fall. This rapid drop in SST coincided with Hurricane Lee’s passage through the area, demonstrating how atmospheric conditions can alter ocean surface conditions.

Figure 5: A timeseries of daily average SST anomalies in the Gulf of Maine (solid colored line) compared to the long-term climatological mean, 10th, and 90th percentiles (solid black lines) in the Gulf of Maine for the period January 1, 2023 through August 31, 2023. Red portions denote when temperatures have exceeded the 90th percentile, the threshold beyond which conditions can start to be categorized as a MHW

Heatmap of Temperature Anomalies and Heatwave Events

Looking at the full record of daily SST anomalies in the Gulf of Maine (Figure 6), the distinct thermal regime shift beginning around 2010 is evident. Indeed, since 2012, the Gulf of Maine has experienced far more persistent MHW conditions (indicated by solid black lines) than at any other point in the satellite record.

Conditions in the Gulf of Maine this fall were unusually cool when compared to recent years. Fall SST in the previous 3 years had been noteworthy for their extended MHW conditions, part of a long-term trend of unseasonably warm fall and fall temperatures since 2012. The relatively mild fall of 2023 stands in contrast to the preceding spring and summer seasons, which were the 2nd & 8th warmest on record.

Figure 6: Heat map of daily SST anomalies from the beginning of 1982 through spring 2023. Not only do more large warm anomalies (darker reds) appear more frequently in recent years, but the frequency, duration, and intensity of marine heatwave events (black lines) in the Gulf of Maine has become more pronounced in the past decade.

Spatial Distribution of Seasonal Anomalies

From an aerial perspective, the Gulf of Maine experienced mild above-average SSTs during fall of 2023`, while surrounding areas experienced either extreme highs or extreme lows for the season. During the fall a large area of below average sea surface temperatures developed just south of Georges Bank. Further to the south a large patch of above-average SST anomalies had formed, located to the east of New Jersey just off the continental shelf and outside the domain analyzed in preceding sections (Figure 7). The highest seasonally averaged SST anomaly of any location in this broader region was 3.05°F - part of the large patch of positive anomalies to the east of New Jersey, south of Long Island Sound.

Figure 7: Map of average SST anomalies for each grid cell in the satellite record for spring 2023. The box outlined by the black dashed line denotes the region of study for the analysis (see Figure 1). Darker red regions indicate warmer anomalies.

Monthly Temperature Anomalies

Average monthly SST anomalies are shown in Figure 8. The warmest anomalies were observed in March and April and were largely confined to areas just beyond the region of study (i.e, beyond the continental shelf where the Gulf Stream and Labrador Current have significantly more influence on oceanic conditions).

Figure 8: This series of maps shows the average monthly SST anomaly for March 2023, April 2023, and May 2023. The box outlined by the black dashed line denotes the region of study for the analysis. Darker red regions indicate warmer anomalies.

North Atlantic Heat

2023 was a year with headline temperatures for the North Atlantic. A dramatic figure that was shared widely on social media was a version of figure 11. which captures just how profoundly hot this year’s temperatures were across the N. Atlantic. SST in the North Atlantic have been off the charts hot for much of the year drawing attention and concerns for the rapidly changing ocean environment.

Figure 9: Light gray lines illustrate annual time series for SST anomalies in the North Atlantic from 1982-2023. North Atlantic SST anomalies for 2023 are shown in orange. Dashed black lines denote the 90th and 10th percentile SST anomalies (for the 1991-2020 CRP).

In a departure from the now-familiar pattern of regionally concentrated warming, much of the attention this year was on large areas in the eastern Atlantic experiencing a large-scale MHW event. This summer scientists sounded the alarm over record setting temperatures for much of the Eastern Atlantic impacting population centers along the European coast and contributing to sweltering temperatures on land.

Figure 10: Map of average SST anomalies for each grid cell in the satellite record for summer 2023 in the North Atlantic. Darker red regions indicate warmer anomalies.

Data Sources:

NOAA_ERSST_V5 data provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their Web site at https://psl.noaa.gov/data/gridded/data.noaa.ersst.v5.html.

NOAA High Resolution SST data provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their Web site at https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html.

Citing This Work

If you would like to cite this report, please use:

Gulf of Maine Research Institute. 2022. Gulf of Maine Warming Update: Fall 2023

A work by Adam A. Kemberling

Akemberling@gmri.org